1. The Field of the Invention
This invention concerns systems and procedures for controlling the temperature of a material to be processed in the atmosphere of a processing chamber which is closed to the surrounding atmosphere, by comparing a desired temperature with the actual temperature in a first and a second control circuit. The actual temperatures are determined by at least two independent temperature sensors in the processing chamber. The temperatures undergo a plausibility test with a specified threshold difference value. Non-fulfillment of the plausibility condition causes a signal to be output and the energy supply for stabilizing the temperature of the processing chamber to be interrupted.
2. Present State of the Art
DE 31 00 273 A1 discloses a failure- and drift-proof temperature regulator in which two sensors determine the actual temperature in the processing chamber of an oven. A comparator then performs a plausibility check, according to which the difference between the two temperature signals must lie within a specified tolerance range for them to be plausible. As a prerequisite, it is assumed that it is extremely unlikely for an error of equal magnitude to occur in two temperature regulators or sensors simultaneously.
This reference also discloses the use of a logic system which tests at least two thermometers and which, if the temperature deviation is too great, not only outputs a warning signal but passes control of the temperature in a processing chamber from a first controller to a second controller. Here, the test characteristics are, e.g., the difference between the expected value and the actual value, the feed current of the instrument, the position of the limit switch in the instruments, and the duration of absence of a new command to change the control variable. Furthermore, the comparator can be blocked during a heating or cooling phase of the heat treatment system.
A temperature control device is also know from DE 82 29 723 U1 with a first thermostatic switch controlled by a first temperature-sensitive element, and a second thermostatic switch controlled by a second temperature-sensitive element. The second temperature-sensitive element can turn on the second thermostatic switch at a certain limit temperature outside the operating temperature range. The second thermostatic switch is arranged next to the first one and has a manual reset component. A display element arranged next thereto indicates its control state. Such a control device, when used on a freezer, is intended to display a warning if the normal control cycle within the operating temperature range has been disturbed, such as, e.g., during a longer power failure.
Here it turns out to be a problem that even when the plausibility condition of the temperature sensors is met, additional defects in the control circuit can occur. Such as, for example, false triggering of actuators. This, for example, can cause a contact to remain permanently closed.
An incubator is also know from DE 21 33 119 B2 which uses water as a circulating fluid and which uses temperature regulation to keep the water at a constant temperature level, such as 37.degree. C., for example. It turns out to be a problem if an error occurs in the heating system. This makes it possible for the material in the useful space, for example cell cultures, to be endangered by overheating, for example. This also goes for falling below the specified temperatures when the heating completely fails. It is also possible for individual elements of the feedback control circuit to fail, for example, the temperature sensor. This can cause an uncontrolled situation which can destroy cell cultures.